Adaptations to Physical Stresses in the Intertidal Zone: The Egg Capsules of Neogastropod Molluscs

Abstract

The encapsulation of eggs within benthic egg capsules or gelatinous egg masses is a common phenomenon among many marine invertebrate groups, yet the functional significance of many aspects of these egg coverings remains unexplored. In this paper I review what is known about the effectiveness of neogastropod egg capsules in protecting embryos from physical stresses associated with the marine intertidal environment. Egg capsules spawned by intertidal neogastropod molluscs can provide embryos with significant protection from desiccation, osmotic stress, and ultraviolet (UV) radiation, relative to embryos devoid of such coverings. Despite this, capsules desiccate rapidly in air, are highly permeable to small solute molecules, and are not impervious to incident UV radiation. Egg capsules of intertidal gastropods are also substantially more permeable to water molecules than the well studied egg cases and egg shells of insects and terrestrial vertebrates and may be no more effective in protecting embryos from such physical stresses than the capsules of exclusively subtidal gastropods. Hence, capsular cases appear to be poorly adapted to protecting embryos from environmental stresses associated with periodic exposure to air. The degree to which the encapsulated embryos of intertidal neogastropods are protected from environmental stresses thus may be more reflective of adult spawning site selection and tolerances of encapsulated embryos to these stresses, than properties of the capsular case, per se. Clearly, however, there is much still to be learned about the protective nature of capsule walls and the tremendous diversity of egg coverings that exists within the Gastropoda.